Q: HOW DO THE 2008 PHYSICAL ACTIVITY GUIDELINES FOR AMERICANS DIFFER FROM PREVIOUS RECOMMENDATIONS? THERE SEEMS TO BE SO MANY OF THEM, AND I AM DEFINITELY CONFUSED.
A: Physical activity recommendations were first published in the early 1970s by the American Heart Association and YMCA, and since then, many other organizations have submitted guidelines including the American College of Sports Medicine (ACSM), U.S. Centers for Disease Control & Prevention (CDC), National Institutes of Health, Surgeon General, U.S. Department of Agriculture, Institute of Medicine, and now the U.S. Department of Health and Human Services (HHS) (2,3).
These reports substantially vary, and you have a right to be confused. The good news is that the 2008 Physical Activity Guidelines for Americans (PA Guidelines) is a superb document that is being viewed as the definitive statement on the amount and type of physical activity people need for health, fitness, and disease prevention.
The HHS released the PA Guidelines to provide science-based guidance to help Americans aged 6 years and older improve their health through appropriate physical activity (3). The content of the PA Guidelines complements the Dietary Guidelines for Americans, a joint effort of the HHS and U.S. Department of Agriculture, and was developed using similar scientific review methods (4).
The 2008 PA Guidelines for adults include these specific recommendations:
* All adults should avoid inactivity. Some physical activity is better than none, and adults who participate in any amount of physical activity gain some health benefits.
* For substantial health benefits, adults should do at least 150 minutes a week of moderate-intensity or 75 minutes a week of vigorous-intensity aerobic physical activity, or an equivalent combination of moderate- and vigorous-intensity aerobic activity. Aerobic activity should be performed in episodes of at least 10 minutes, and preferably, it should be spread throughout the week. These guidelines also apply to older adults but should be adapted as their abilities and conditions allow. Older adults also should do exercises that maintain or improve balance if they are at risk of falling. When adults with disabilities are not able to meet these activity guidelines, they should engage in regular physical activity according to their abilities and avoid inactivity. Adults with chronic conditions obtain important health benefits from regular physical activity but should be under the care of a health care provider.
* For additional and more extensive health benefits, adults should increase their aerobic physical activity to 300 minutes a week of moderate-intensity or 150 minutes a week of vigorous-intensity aerobic physical activity, or an equivalent combination of moderate- and vigorous-intensity activity. Additional health benefits are gained by engaging in physical activity beyond this amount.
* Adults also should do muscle-strengthening activities that are moderate or high intensity and involve all major muscle groups on two or more days a week because these activities provide additional health benefits. The time spent in muscle-strengthening activities can be added to the weekly tabulation of physical activity minutes. No specific recommendations for flexibility exercise were advanced, but people are still encouraged to stretch because the increase in flexibility can allow people to "more easily do activities that require greater flexibility" (3).
A unique feature of the PA Guidelines is the division of bodily movement into two categories called baseline activity and health-enhancing activity. Baseline activity is defined as the light-intensity activities of daily life, such as standing, walking slowly, lifting lightweight objects, and very short episodes of moderate- or vigorous-intensity activity such as climbing a few flights of stairs.
Health-enhancing physical activity is defined as activity that when added to baseline activity produces health benefits (see Table for a list of unique definitions used in the PA Guidelines). Some workers such as postal carriers, construction workers, and landscape gardeners may get enough physical activity on the job. Although the PA Guidelines do not include baseline activities in the recommended minutes per week of activity, they should not be discouraged because they increase overall energy expenditure, help maintain a healthy body weight, and some are weight bearing and may improve bone health.
The 2008 PA Guidelines define being inactive as no activity beyond baseline activities of daily living, low activity as fewer than 150 minutes of moderate-intensity physical activity a week or 75 minutes of vigorous-intensity activity, medium activity as 150 to 300 minutes of moderate-intensity activity a week or 75 to 150 minutes of vigorous-intensity physical activity a week. High activity is defined as more than the equivalent of 300 minutes of moderate-intensity physical activity a week.
In scientific terms, the medium activity range is approximately equivalent to 500 to 1,000 metabolic equivalent (MET) minutes a week. A metabolic equivalent, or MET, is the ratio of the rate of energy expended during an activity to the rate of energy expended at rest. For example, 1 MET is the rate of energy expenditure while at rest. A 4-MET activity expends four times the energy used by the body at rest. If a person does a 4-MET activity for 30 minutes, this is calculated as 4 × 30 = 120 MET minutes of physical activity. A person also could achieve 120 MET minutes by doing an 8-MET activity for 15 minutes.
Light-intensity activities are defined as 1.1 METs to 2.9 METs, moderate-intensity as 3.0 to 5.9 METs, and vigorous-intensity as 6.0 METs or more. Running at 10 minutes per mile (6.0 miles per hour) is a 10-MET activity and is therefore classified as vigorous intensity, and walking at 3.0 miles per hour requires 3.3 METs of energy expenditure and is therefore considered a moderate-intensity activity.
A key recommendation of the 2008 PA Guidelines is that the health benefits of physical activity depend mainly on total weekly energy expenditure caused by physical activity (i.e., 500-1,000 MET minutes per week). A range is necessary because the amount of physical activity necessary to produce health benefits varies by the health benefit. For example, activity of 500 MET minutes a week lowers the risk of premature death, but activity of more than 500 MET minutes a week is necessary to reduce the risk of breast cancer. Metabolic equivalent minutes is a difficult concept for the average American adult to understand, and the PA Guidelines committee decided to translate this into minutes per week (i.e., 150-300 minutes per week of moderate-intensity activity).
The 1995 CDC-ACSM recommendation that adults "should accumulate 30 minutes or more of moderate-intensity physical activity on most, preferably all, days of the week" was misinterpreted by some who believed that the daily activities of living were sufficient for health and fitness (1) The PA Guidelines also describe the CDC-ACSM guidelines as too specific. For example, scientific evidence does not support that the health benefits of 30 minutes on 5 days a week are any different from the health benefits of 50 minutes on 3 days a week. As a result, the new PA Guidelines allow a person to accumulate 150 minutes a week in various ways, with the proviso that the bouts be spread throughout the week and last longer than 10 minutes each.
Children and adolescents should do 60 minutes or more of physical activity daily, with an emphasis on enjoyment and appropriate adjustments for each age level. Most of the 60 or more minutes a day should be either moderate- or vigorous-intensity aerobic physical activity. As part of their 60 or more minutes of daily physical activity, children and adolescents should include muscle- and bone-strengthening physical activity on at least three days of the week.
The 2008 PA Guidelines also emphasize that many adults will need to do more than 150 to 300 minutes a week of moderate-intensity aerobic physical activity to lose weight or keep it off. Combined with restriction of caloric intake, overweight adults should gradually increase the minutes or intensity of aerobic physical activity per week, to the point at which the physical activity is effective in achieving a healthy weight.
Overall, the 2008 PA Guidelines are a historic and landmark document that go beyond and supersede all previous published recommendations. The PA Guidelines are both scientifically sound and easy to communicate and greatly simplify the essential issues. ACSM and other organizations have already adjusted their exercise prescription guidelines in accordance with the PA Guidelines.